If either tv or tz is NULL, the corresponding structure is not set
or returned. (However, compilation warnings will result if tv is NULL.)

The use of the timezone structure is obsolete; the tz argument
should normally be specified as NULL. (See NOTES below.)

Under Linux, there are some peculiar "warp clock" semantics associated
with the settimeofday() system call if on the very first call (after
booting) that has a non-NULL tz argument, the tv argument is
NULL and the tz_minuteswest field is nonzero. (The tz_dsttime
field should be zero for this case.) In such a case it is assumed that the
CMOS clock is on local time, and that it has to be incremented by this amount
to get UTC system time. No doubt it is a bad idea to use this feature.

The time returned by gettimeofday() is affected by discontinuous
jumps in the system time (e.g., if the system administrator manually changes
the system time). If you need a monotonically increasing clock, see
clock_gettime(2).

Macros for operating on timeval structures are described in
timeradd(3).

On a non-Linux kernel, with glibc, the tz_dsttime field of struct
timezone will be set to a nonzero value by gettimeofday() if the
current timezone has ever had or will have a daylight saving rule applied. In
this sense it exactly mirrors the meaning of daylight(3) for the
current zone. On Linux, with glibc, the setting of the tz_dsttime field
of struct timezone has never been used by settimeofday() or
gettimeofday(). Thus, the following is purely of historical interest.

On old systems, the field tz_dsttime contains a symbolic constant (values
are given below) that indicates in which part of the year Daylight Saving Time
is in force. (Note: this value is constant throughout the year: it does not
indicate that DST is in force, it just selects an algorithm.) The daylight
saving time algorithms defined are as follows:

Of course it turned out that the period in which Daylight Saving Time is in
force cannot be given by a simple algorithm, one per country; indeed, this
period is determined by unpredictable political decisions. So this method of
representing timezones has been abandoned.

This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at
https://www.kernel.org/doc/man-pages/.